The present invention relates to a device for velocity matching between optical and electrical signals in a waveguide structure comprising first waveguiding means for optical signals and second waveguiding means for electrical signals.
It is a problem with electro-optical devices based on waveguiding structures (Travelling Wave structures) that the band-width is limited by so called walk-off, i.e. an electrical signal and an optical signal propagate with different velocities or group velocities. In the case of a modulator modulating and modulated signals thus propagate with different velocities.
For example, in fast digital fibre-optical systems, particularly with bit velocities of more than 2,5 Gbit/s, it is important to limit walk-off. In order to reach these high bit velocities with optical signals fast optical modulators are required whereby either fast direct modulating lasers or fast electro-optical modulators can be used. The lasers are cheaper but are a qualitively inferior solution than the electro-optical modulators and with very high bit velocities, more than approximately 10 Gbit/s, it is today necessary to use electro-optical modulators.
Also in analog fibre-optical systems which e.g. may have a bandwidth of some GHz it is interesting to be able to reduce walk-off, which in such systems case above all gives rise to distortion.
Apart from waveguide losses walk-off is a limiting factor for a large group of waveguide modulators. Whereas waveguide losses is a physical phenomenon which above all is due to dimensions and resistivity of the waveguide, where the losses may be reduced by high fabricational requirements and high requirement as to the surface and bulk properties of the electrode (elimination of waveguide losses means in principle that suprerconducting electrodes are used), walk-off can be seen as a design problem. E.g for LiNbO.sub.3 -modulators walk-off means that the group velocity of the modulating, electrical signal is less than the group velocity of the modulated, optical signal.
A number of solutions have been suggested to reduce or minimize walk-off or to match the velocity between optical and electrical signals. Therethrough the so called V/GHz-quotient can be lowered. Solutions have also been suggested to minimize the V/GHz-quotient, among others walk-off has been used in order to increase the bandwidth and furthermore the electrical and optical field pattern have been adapted to each other to reduce the switching voltage.
EP-A-0 152 996 describes a device for matching the velocity between an optical and an electrical signal with a thick and etched buffer layer which is invariant in the direction of propagation. Therethrough a certain velocity matching and an increased bandwidth is achieved, but this particularly at the cost of a too high feeding voltage.
In U.S. Pat. No. 4,468,086 velocity matching is simulated between an electrical and an optical signal through the electrical signal path being so formed that interaction between the two waves occurs along selected regions of the path of propagation of the electrical signal; the matching is simulated through a phase delay. Particularly the electrical path is bent away from the optical path at uniformly spaced intervals in order to create areas where no interaction occurs. Therethrough optical waves are modulated by the electro-optical effect.
Through U.S. Pat. No. 4,448,479 the walk-off effect is to be minimized through exposing, at uniformly spaced intervals, the electro-optical induced TE-TM-coupling coefficient to a phase shift of 180.degree., also in this case is thus a velocity matching simulated, in this case through phase reversal. In as well U.S. Pat. No. 4,468,086 as U.S. Pat. No. 4,448,479 the velocity matching can be used to make the structure "resonant" and therethrough the bandwidth can be increased, but at the same time, with a great probability, the phase response of the modulator will be destroyed, wherethrough it cannot be used for digital communication.
Particularly with LiNbO.sub.3 -modulators, essentially generally with a lowering of walk-off, a part of the modulating (electrical) signal is led in LiNbO.sub.3 and may propagate in a different material which has a lower dielectric constant and in this way the group velocity of the electrical signal is increased. LiNbO.sub.3 has dielectric constants of 28 and 43 respectively in different directions (the material is anisotropic); in certain models however the material is treated as isotropic and then the geometrical average of 34,7 is used. The square root of the dielectric constant is to be compared with the refractive index of LiNbO.sub.3 which is about 2,2; thus the difference in group velocity between the electrical and the optical signal is great. For coplanar waveguides the group velocity of the electrical signal is not less than c/4,2 and a number of devices are known of which the group velocity is increased by use of thick electrodes, thick buffer layers and with etched buffer layer of e.g. SiO.sub.2. Further devices comprise a "parallel" waveguide of SiO.sub.2 or air as dielectricum wherein also the "parallel" waveguide may comprise a groundplane of its own. Generally a considerably increased feeding voltage is required with known solutions in order to obtain an increased bandwidth.